Best way to store/initialize a persistant variable in Node-Red block?

I am using node-red and I would be interested on making a persistant varible ( atcually an array), that is not deleted each time that data reached a node. I have read about context option but I cannot manage to make it work.

The point is that I have a block where I woould like to store some incoming data, so later I can compare that stored data with the new incoming one. However, if I define a variable in such a block, that variable will be overwritten each time that something arrives to the block itself, so I cannot effectively compare.

How can I do that then? And, what/where is the best way to initialize that variable?